1. Field of the Invention
This invention relates to hydraulic platform lifts for use on truck trailers and the like for loading or unloading cargo when no loading dock is available.
2. Description of the Prior Art
Previous lift platforms are generally of two types. A first type is often referred to as a TUK-A-WAY (a trade mark) where the platform is tucked away underneath the rear end of the trailer when not in use and through hydraulic means it can emerge from beneath the trailer for use in raising or lowering cargo. The TUK-A-WAY lift has one hydraulic cylinder which can be used to provide a positive force to the platform in an upward direction but no force is applied to the platform in the downward direction. The platform simply lowers by gravity as the hydraulic pressure is released. The TUK-A-WAY lift cannot be used where the rear axle of the trailer is located close to the rear of the trailer (as is often the case) as there is then no space for the TUK-A-WAY to be stored when it is not in use. A second type of lift has a horizontal hydraulic cylinder that is used in conjunction with several complex connectors and cables to move a platform located at the rear of a trailer upward. This type of platform lift also moves the platform downward by gravity as hydraulic pressure is released. There is no positive force moving the platform downward. This type of lift stores the platform in an upper vertical position when the truck is travelling down a highway. The platform can be moved into an upper horizontal position, a lower horizontal position or a lower vertical position for loading or unloading cargo, as desired. In the travel position, the cylinder is extended and is thus exposed to the elements, such a road salt. With previous lifts, since there is no positive downward force that can be exerted on the platform hydraulically, if the lift is not working perfectly, the platform will often not lower unless it is loaded with cargo. Further, if one of the hydraulic lines breaks when the platform is in an upper position, it will crash to the ground, with or without cargo as the hydraulic pressure is released. Obviously, personal injury or damage to the cargo can result. Previous platform lifts are too complex, subject to frequent mechanical failure or too expensive to manufacture. Further, with previous platform lifts, when the lift is located at the rear of the lead unit of a B-train, many lifts are completely unsuitable as they prevent the connection of the fifth wheel to the B-train trailer so that the rear trailer can be connected. A B-train is a trucking system where a tractor pulls two trailers simultaneously. The pivot pin of the second trailer rests on a fifth wheel secured to the rear of the first trailer. Previous platform lifts are designed for a trailer of a specific width and cannot be easily adapted to a trailer with a different width.